The Mast

On modern sailboats the main purpose of a mast is to serve as a structural component in a "wing". There are five building materials of which I know mast have been built, only two are practical. Aluminium alloys are the most popular, these are akin to the alloyes seen in aircraft and are very different from the marine grade aluminum seen in boat hulls. Carbon fiber is slowly replacing aluminum as the material of choice, it is stronger, lighter and stiffer than aluminium. The not so practical category includes wood, steel and fiber glass, before the 1950s wood was the most popular material. In the age of sailboats the masts were made from a single spruce, it would often take months to find a tree with the right properties/height. In recent years most wooden masts are of a combosite construction with modern super-glues. Wood has serveral disadvantages, its relatively heavy, it rots, is high maintenance and is weaker than modern materials. Steel masts are commonly seen on commercial vessels, although these do not carry sails. The only rig I would even consider a steel mast for is a Gaff rig and even then aluminium is usually a better choise. The problem with steel as a mast material is weight, it puts alot weight high up where you least want it. Fiber Glass is the worst mast material of all, it's heavy, to flexible and when subjected to repeated flexing load has a high risk of delaminating, don't even think about it*! The only mast building material which I have not mentioned but would be very suitable is alloyed titanium. Titanium alloys have the best strengh to weight ratio of any metal and the best corrosion resistance, they are however to expensive to even consider.

Both Carbon Fiber and Aluminium also have severe shortfalls but we have learned to live with these. The biggest problem with the Aluminium alloys used for aircrafts/masts is it's low weldability, a weld in these alloys will never have more than 60% of the original material strengh. This is the reason why all aircraft are riveted and why everything is riveted/bolted onto masts. This is also the reason why 99% of masts are "seamless" extrusions, the form is created during the solidificatin of the metal and cannot be altered after manufacturing without a major stength loss. The problem with extrusions is that it is impossible to make tapered masts, hence your mast is the same shape from top to bottom even though from a design standpoint it would be better to have a tapered form. Repairing a damaged aluminium mast will never yield good results, the only temporary solution is to put a "sleeve" inside or outside the mast and rivot the whole thing togeather. Carbon fiber has it's own set of problems, like aluminium it is very hard to alter the form or add to the design after the solidification of the resin**. Carbon Fiber is impossible to repair, if it breaks replace it. Asside from a better strength to weight ratio than aluminium it also can be produced in tapered forms reducing the weight high up and making these very popular in the racing community. Another problem with carbon is it's nobility on the galvanic scale***, if it is submerged while in electrical contact with other metals it will cause extreeme corrosion, this is especially a problem with aluminium hulls which are base.

Mast strength is generally expressed as Moments of Inertia which is just a fancy way of expressing an objects resistance to bending or simply put it's stiffness/stength in relationship to length.**** Since most masts today are elliptic they actually have a range of intertias but these are represented by the lowest/side and highest/front in the form 390x970cm4 (9,37x23,2in4). The intermediate values are irrelevant as is the "backwards" load since the side/forward loads will generate a spar that ensures ample strength in these directions. For the Hout Bay 33 the moments are: 390x970cm4 (9,37x23,2in4), this and the length of 14.3m (46'11") is everything I need to know about the mast properties.

Once the mast is stepped it needs to be tuned with the rest of the rig, this includes inducing both rake and bend to the mast, for more information there are plenty of good guides available on the internet.******

Fasteners used with masts should exclusively be stainless type 316, if this is ignored for aluminium or carbon masts severe galvanic corrosion can occur. It is higly recomended to use specialized fittings/fasteners for carbon masts.

*Fiber Glass and Carbon Fiber use quite similar construction but vary greatly in strengh, weight and quality.

** Carbon fiber like Fiber glass is made of a fiber and a resin, the fibers which are very strong are suspended in cured resin which makes them rigid.

*** Galvanic scale, see Wikipedia for Galvanic Scale

**** Online calculators for Moments Tube and Pipe

***** A neutral helm is when a boat does not round up or give away

****** Sailing Supply | North Sails 505 Tuning guide | Ballenger Spars